A high-quality optical local oscillator OLO or a high-quality optical signal are necessary in methods or devices used for optical linear sampling OLS. The quality of the optical local oscillator OLO and the optical signal may be measured in terms of optical spectral line width. The quality of such an optical signal may also be measured in an equivalent fashion by its coherence time. The best-quality optical signals have a narrower line width or a longer coherence time. Although it is possible to implement a high-quality optical local oscillator (with greater costs), the input optical signal has no reason to be high-quality, because most of the time the optical signal OS, which the OLS systems must measure, is unknown.
One possible route is improving OLS systems such that the poor quality of the input optical signal no longer has any impact on the resulting output optical signal.
A known first solution consists of duplicating the layout of the OLS system by introducing a one-bit delay between the optical signal entering the original OLS system and the optical signal entering the duplicate system. One of the main drawbacks of the solution is that the one-bit delay must be readjusted to the binary bitrate of the input optical signal. This therefore limits its practical application, unless automated detection of the binary bitrate is implemented, and the one-bit delay is automatically readjusted.
Another known solution is based on simultaneous optical linear sampling of a continuous optical signal or CW (for “Continuous Waveform”) signal and of a modulated optical signal which has been obtained from that incoming CW optical signal. The measurement is carried out such that the two optical linear sampling efforts also require a duplication of the OLS system. However, in this solution, no readjustment of the delay is performed. The primary drawback of this solution is that it is not appropriate for measurement of any unknown input optical signal by the OLS technique, because the modulated optical signal is produced within the experimental device. However, the solution is highly robust with respect to the respective quality of the modulated optical signal or the pulsed optical signals of the sampling.